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PWA windows are intended to appear stand alone from the main
browser so they should not adopt the main browser's theme colors.
This CL provides a default theme getter to be used by hosted app
(and PWA) windows.

This CL also removes the quick fix for this issue done by
https://chromium-review.googlesource.com/c/chromium/src/+/1275466.

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=363145&signed_aid=TMzuMVlMKC2dm6ppdYp9PQ==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=363146&signed_aid=Og46P66KeV7iP-mnBHO3cA==&inline=1

Bug: 891560
Change-Id: I2d94bff9f72bfca59fcf7d6cb940f42e4a25cc70
Reviewed-on: https://chromium-review.googlesource.com/c/1272896
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600643}